1. Field of the Invention
This invention generally relates to a method of fabricating a thin film transistor liquid crystal display (“TFT-LCD”), and more particularly to a method of fabricating a polysilicon thin film for the thin film transistor array in the liquid crystal display.
2. Description of the Related Art
Polysilicon and amorphous silicon are two types of materials used for fabricating TFT for active matrix LCD panels. Polysilicon TFT has a higher aperture rate and a lower cost than the amorphous silicon TFT. The larger and uniform silicon grains of polysilicon allow electrons to flow more freely than through amorphous silicon, which is made up of smaller and random sized silicon grains. It allows the normally external driver chips to be fabricated on the glass substrate that dramatically reduces row and column connections. Hence, polysilicon TFT technology can effectively reduce the device size in order to achieve higher integration. Generally, the requirements for mass production of polysilicon TFT are low temperature polysilicon (“LTPS”) technology (450-550° C.), low temperature film-forming technology for the isolating film of gate terminal, and ion implantation for large area.
For application to active matrix liquid crystals displays, a low temperature process for the production of polycrystalline silicon is required to permit the use of inexpensive glass substrates. This would allow the integration of drive electronics into the display panel. Current low temperature process include solid phase crystallization (“SPC”) and excimer laser crystallization (“ELC”). Solid phase crystallization requires high temperatures (600° C.) and the result of the crystallization is not very good. Excimer laser crystallization technology applies the excimer laser is excimer laser crystallization or excimer laser annealing (“ELA”) process to fuse the amorphous silicon thin film and make it recrystillize to a polysilicon thin film.
Because the ELC process can be carried out at a temperature lower than 450° C., resulting in a higher electron migration rate and a lower leakage current than the SPC process, it can be applied to an inexpensive glass substrate. Therefore, the production costs can be reduced.
FIGS. 1A-1B show the fabrication process of a conventional polysilicon thin film. Referring to FIG. 1A, a substrate 100 having an isolating layer 102 formed thereon is provided. Then a smooth amorphous silicon layer 104 is formed on the isolating layer 102. After the amorphous silicon layer 104 is deposited, the excimer laser 106 with enough energy is applied to substantially fuse the amorphous silicon layer 104; i.e., there are some unfused silicon grains left on the surface of the isolating layer 102 as discrete seeds (not shown in FIG. 1A).
Referring to FIG. 1B, the fused amorphous silicon layer 104 uses the unfused silicon grains as discrete seeds to crystallize into a polysilicon layer 108, which is used for drain region, source region, and channel region of the TFT.
The conventional process of forming polysilicon thin films has to precisely control the power of the excimer laser in order to make some unfused amorphous silicon grains left as discrete seeds for better crystallization. However, the excimer laser is a pulsed laser. The power density of each pulse would be different and thus is difficult to control. As a result, the size of silicon grains left would be different; the silicon grains would comprise protrusions at their boundaries reflecting a poor smoothness of the polysilicon thin film, which would adversely affect the TFT devices.